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Re: DAC I/V conversion

82.35.157.35

Posted by Kuei Yang Wang on June 26, 2004 at 12:49:49

In Reply to: DAC I/V conversion posted by Pooge on June 25, 2004 at 09:05:43:

Konnichiwa,

> but in the past, current output DACS required a zero voltage
> load for minimum distortion,

That is not entierly true. A R2R Ladder DAC can be loaded with a resistor of the Value R or lower (usually in the KOhm range!!!) to give best linearity. A very specific exception are the Philips "multibit" DAC's (TDA1540/1541/1543/1545), which do not use R2R structures, the usual AD and BB Multibit DAC's use R2R, as do the BB "mixed mode" DACs, such as used by the RAKK. These DAC's in effect delta-sigma modulate a multibit R2R converter.

So all of the modern Multibit and mixed mode DAC's can tolerate significant resitive loads without loosing linearity, HOWEVER, their outputs tend to be "clamped" with diodes, which usually limit the analogue voltage to around 0.6V before severe distortion occurs, due to the diodes litereally clipping the voltage across the resistor.

> A transimpedance amp has been promoted by Hawksford and in
> I/V discussions on diyAudio forums.

As always, there are many ways to do a given task. I have myself promoted a transimpedance Amp (OPA660) with DAC's but am equally fond of the simple resistor with a nice valve stage. Even using superfast current feedback Op-Amp's as I/V has many good properties. What one specifically employs tends to be a result of thendesigners preferences and prejudices, experiences and commercial (or not) considerations, including the presence of a "Unique Selling Point".

Sayonara

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